Q: cannot increase brightness

The following steps may help:

• Restart both your iPhone and your watch, turning both devices off together, then restarting your iPhone first;
  • Restart your iPhone, iPad, or iPod touch - Apple Support
  • Restart your Apple Watch - Apple Support

• Unpair and re-pair your watch:
  • The Watch app on your iPhone takes a backup of your watch automatically when unpairing via the app. Choose to Restore from Backup during set up. Most data and settings will be restored, with some exceptions (eg Apple Pay cards, passcode).
  • Unpair your Apple Watch and iPhone - Apple Support
  • Set up your Apple Watch - Apple Support

If both devices are already up to date with the latest software, then I suggest that you contact Apple Support (mail-in service may be available) or make a Genius Bar reservation with a view to having your watch checked and, if recommended by Apple, serviced (whether under warranty or via out-of-warranty repair service):

• Contact - Official Apple Support
• https://twitter.com/applesupport
• http://www.apple.com/retail/geniusbar/

To check your warranty status (the serial number is on the back of your watch):

• https://checkcoverage.apple.com/

You can increase the brightness on your watch in the settings menu on your watch. Push the crown and go to settings/brightness-text size. Hope this helps